The invention relates to magnetic storage devices using transducers or recording heads for reading, writing and erasing data on a movable magnetic storage medium. The data are typically arranged in parallel tracks such that the transducer follows an individual track as the storage medium is moved. For example, in a rotatable disk the data are arranged in concentric tracks with the transducer radially movable with respect to the disk.
As data density increases and the space between adjacent tracks is correspondingly reduced, it becomes advantageous to record transducer positioning or servo data on the recording medium, thus permitting more accurate positioning than is possible using an entirely mechanical means. An entire disk surface can be dedicated to servo positioning data, as shown in U.S. Pat. No. 3,864,740 to Sordello et al granted Feb. 4, 1975. Alternatively, servo data can be written in sectors on the same medium containing the working data. This technique, called embedded servo and disclosed in U.S. Pat. No. 3,185,972 to Sipple, granted May 25, 1965, requires comparatively less media storage space and eliminates the need for a separate servo transducer.
Transducer positioning using servo data occurs in two stages: coarse positioning or track seeking, in which the head is directed from a random location to a selected track; and fine positioning or track following, during which the transducer, already near the selected track, is maintained in a centered position with respect to the track.
U.S. patent application Ser. No. 257,297 filed Apr. 24, 1981 (the West et. al. Application) discloses an improved servo positioning system and is incorporated by reference in this application. One of the features disclosed in the West et. al. Application is the use of automatic gain control (AGC) data in each servo track to set a threshold for recovery of coarse positioning data in the same servo track. Pulses having peak amplitudes less than the threshold value are not detected, thus to avoid an erroneous recognition of noise as coarse positioning data. The disk drive embodying this servo positioning system further utilizes improved, lighter head loading to avoid the catastrophic head crash of previous designs.
However, the lighter head support does not avoid a different problem, namely, the gradual deterioration of signal amplitudes. Such damage is difficult to detect, first because it does not readily appear upon visual inspection. Secondly, as the recovery threshold depends upon AGC data--usually it is set at fifty percent of the average AGC peak level--the threshold is reduced along with AGC and servo positioning data as they deteriorate. Thus, coarse positioning data, if degraded at the same rate as AGC data, continues to be detected due to the reduced threshold, eventually to the point of data loss due to lack of AGC dynamic range and, occasionally, media or drive failure.
Testing for errors, whether in media or drive mechanisms, is well known in magnetic data recording. U.S. Pat. No. 3,506,814 to MacDonald et. al., granted Apr. 14, 1970, shows a test tape 16 for determining, among other things, the tape drive failure point for weak signals. Characters are written in selectively reduced amplitude ranging from seventy to ten percent design amplitude. Error is indicated when the read apparatus fails to pick up a signal. Another test is disclosed in U.S. Pat. No. 3,864,741 to Shwarz, granted Feb. 4, 1975, wherein a head normally is centered between two servo tracks and thus receives two servo signals, S.sub.1 and S.sub.2. These signals should be equal when the head is centered, but may not be due to component errors. To determine such errors, disk 17 has an equalization sector 40 which first provides only S.sub.1, then only S.sub.2. Should the equalization sector signals be unequal, S.sub.2 is adjusted as required.
Media testing is shown in U.S. Pat. No. 4,214,280 to Halfhill et. al., granted July 22, 1980. In a disk having data and servo sectors, a test pattern is recorded for each track. The read signal from each track activates a counter whenever its amplitude falls below a select value to indicate a defect, and the defect address is stored in memory. Subsequent write circuitry, upon sensing the defect, activates a two byte delay to prevent writing of data in the defective area.
None of these prior art methods is particularly well suited to anticipate isolated, gradual degrading of data, the problem to which the present invention is directed. It has been found that while it is possible for an entire surface to deteriorate, data tends to degrade most rapidly on a particular track and over its entire circumference. This phenomenon appears to be related to hardware malfunctions causing excessive air-system contamination or magnetic particle contaminated hardware in which the destructive effect is concentrated, by the head dwelling in the same location on the media.
It is an object of this invention, therefore, to provide an inexpensive means for verifying the integrity of servo data. It is another object of this invention to provide a means for early detection of servo signal deterioration, thus to prevent loss of data and damage to media and data reading and recording apparatus. Yet another object is to identify the nature of an impending failure in order to indicate preferred corrective measures.